Gas analyzers are used in medical and industrial environments to analyze the composition of gas samples, e.g., to determine the presence and/or concentration of components of interest in such samples. In the medical environment, gas analyzers are used, for example, to monitor the administration of anesthesia during medical procedures. The components of interest that are monitored in such applications may include anesthetic agents such as isoflurane, enflurane, halothane, desflurane and sevoflurane and respiratory gases such carbon dioxide. Various other therapeutic gases may also be monitored depending on the application. The gas analyzer may operate on gas within the main patient respiratory stream, or on a side stream diverted from the main respiratory stream.
Various types of gas analyzers are used in connection with monitoring administration of anesthesia including spectrographic gas analyzers, Raman scattering gas analyzers, and of anesthesia including spectrographic gas analyzers, Raman scattering gas analyzers, and electrochemical gas analyzers. Spectrographic gas analyzers generally include an illumination source and an illumination detector. Illumination from the source is transmitted through the gas sample and is measured by the detector. Composition information is determined based on known transmissions/absorption characteristics of the components of interest. Raman scattering spectrometers involve transmitting coherent illumination through the gas sample and detecting scattered illumination. Composition information is determined based on known scattering characteristics of the compositions of interest. Electrochemical gas analyzers involve contacting the sample gas with an electrochemical cell or cells and measuring resulting electrical signals. Composition information is determined based on known electrochemical characteristics of the components of interest.
The processing to determine composition information based on analyzer output typically involves calculating a value related to the analyzer output and determining composition information corresponding to the value based on a database of empirically derived information concerning the components of interest. In order to ensure accuracy, certain system parameters such as illumination source intensity or detector temperature may be monitored and changes in such system parameters may be accounted for in making composition determinations. Certain sample gas parameters such as pressure may also be taken into consideration. Other parameters, such as humidity, have generally not been taken into consideration in making composition determinations or designers/operators have attempted to control or limit values of such parameters without active monitoring through the use of filters, humidifiers or the like. Such attempts to control or limit humidity in connection with a patient respiratory stream have proved problematic and have not been entirely successful.